It is well known that determining the frequency response of a control system can help identify problems which may arise during system operation. In the context of magnetic disk drives, however, the frequency response of the servo control system has generally been measured by external test equipment that costs thousands of dollars. The tests, moreover, must occur in a clean room environment because it requires invasive access to the drive's interior. Finally, it may take several minutes to several hours to complete the frequency characterization of a single drive using such external test equipment. Because of these limitations, the drive industry generally tests the frequency response of only a limited number of drives during product development.
During manufacture, each production drive is simply assumed to have a frequency response that is sufficiently "close" to the nominal disk drive(s) tested during development to make the drive operable. There is no mechanism to validate the frequency response on a drive by drive basis. There is conventionally no mechanism to vary the frequency response to salvage a marginal drive individually.
Unfortunately, the frequency response of each production drive may vary from the nominal response of the development drives due to an unpredictable combination of characteristics including the electromagnetic response of each individual read transducer, each transducer's location on the head stack, mechanical misalignments, and electronic component variations. Moreover, circumstances which cause deviations from the nominal frequency response are also likely to alter the magnitude and location of resonances and, thereby, reduce the servo system's gain margin, phase margin, or both, to the detriment of the servo system's operating stability.
The drive industry is highly competitive such that there is a clear need to reduce the percentage of customer returns and obtain a higher overall yield of disk drives in the mass production environment.